In recent years, in the field of light-sensitive silver halide color photographic materials, there are much severer demands for high sensitivity and high image quality.
To answer such demands, a variety of researches have been focused on improvements in silver halide emulsions.
For example, U.S. Pat. No. 4,439,520, Japanese Patent Publications Open to Public Inspection [herein after referred to as Japanese Patent O.P.I. Publication(s)] No. 99751/1987 and No. 115435/1987 disclose a multi-layer light-sensitive silver halide color photographic material having a high sensitivity and a superior sharpness of dye images, in which a tabular grain silver halide emulsion with a grain thickness of 0.3 .mu.m or less and an aspect ratio of 8 or more is used in a high-sensitivity layer.
Japanese Patent O.P.I. Publications No. 93344/1982, No. 145135/1979 and No. 151944/1982 also disclose techniques to improve sharpness by the use of a diffusible DIR compound.
Here, the approach from the tabular grain could be grouped as a technique to improve sharpness mainly by virtue of an optical effect, and the approach from the DIR compound as a technique to improve it by virtue of a development effect.
The light scattering due to silver halide grains has a great influence on the sharpness. As disclosed in the above publications, it is certainly effective in the sense of an improvement in sharpness that the grains are made to have a tabular form and have a thickness of 0.3 .mu.m or less.
However, such tabular silver halide grains have an aspect ratio which is as high as 8 or more, and hence are very sensitive to stress, resulting in a great deterioration of pressure characteristics.
The term "pressure characteristics" has two meanings, one of which is what is called fogging by pressure, which is development of unexposed areas, caused by application of a pressure to light-sensitive silver halide photographic materials, and the other of which is what is called desensitization by pressure, which is a decrease in sensitivity at the time of exposure, or a decrease in density because of a pressure applied after the exposure.
As a matter of course, the pressure characteristics more deteriorate as tabular grains have a smaller thickness, because of their mechanical weakness. When, however, tabular grains have the same thickness, the pressure characteristics deteriorate as they have a higher aspect ratio. This can be explained to be due to the fact that a larger stress tends to be applied to tabular grains than to spherical grains even when materials have the same mechanical strength.
Even if grains having a relatively low aspect ratio are used in the case where all tabular grains contained in a light-sensitive layer are made to have a thickness of 0.3 .mu.m or less in order to improve sharpness, the pressure characteristics can not still be improved when the halogen composition inside the grains is of uniform structure. As a means for improving the pressure characteristics, what is considered as one means is a method in which the halogen composition of silver halide grains is made to be of core/shell structure.
For example, Japanese Patent O.P.I. Publications No. 99433/1984, No. 35726/1985 and No. 147727/1985 disclose techniques by which the pressure characteristics have been improved using a core/shell emulsion comprising a silver halide grain having in its interior a core having a silver iodide content of 10 mol % or more. Follow-up of these techniques by the present inventors, however, has revealed that a great deterioration of graininess may result because of a broad grain size distribution and also because of a broad distribution of silver iodide content in individual grains.
As techniques to better prevent deterioration of graininess, for example, Japanese Patent O.P.I. Publications No. 151618/1988, No. 213637/1989 and No. 838/1990 disclose monodisperse tabular grains mainly having a hexagonal form.
These tabular silver halide grains that can prevent deterioration of graininess, however, have no aforesaid core/shell structure even though they comprise silver bromide or silver iodobromide, resulting in a deterioration of the pressure characteristics. Japanese Patent O.P.I. Publication No. 209445/1987 also discloses an emulsion of 5 or more in aspect ratio and 20% or less in relative standard deviation of average silver iodide content. Japanese Patent O.P.I. Publication No. 213845/1991 still also disclose a tabular grain emulsion having an aspect ratio of 3 or more and a grain thickness of about 0.3 .mu.m and having been prepared through the step of forming grains by feeding fine silver iodide grains. Following-up of these techniques by the present inventors, however, has revealed that the pressure characteristics have been improved with insufficiency.
As discussed above, it has been difficult for the prior art to improve the pressure characteristics and the sharpness at the same time without causing the deterioration of graininess.